A lithographic process is a manufacturing process in which the lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. The patterning step performed by the lithographic apparatus is just one step in a sequence of processing steps performed on each substrate in the entire lithographic process. The processing steps generally including one or more pre-patterning process steps and one or more post-patterning process steps. Examples of pre-patterning steps include steps for applying or modifying layers of product material or mask material, applying a base anti-reflection coating (BARC) and applying a radiation-sensitive resist. Examples of post-patterning process steps include developing the resist, etching a product material or mask material in accordance with the pattern, removing resist, cleaning and so forth. Each substrate may pass through many cycles of patterning steps and processing steps, to build up a desired product structure. Each of the steps involves one or more handling operations, in addition to the chemical and/or physical processes of the steps themselves. Any of these handling operations can introduce defects to the substrate, which influence the performance of subsequent processing steps. Defects may consist of damage to the material of the substrate, or particles of contaminant material adhering to the substrate. Contamination can be transferred from a substrate to the substrate support or other handling apparatus, affecting processing of other substrates in due course.
Performance of the lithographic process can be measured by various parameters. A particular performance parameter known as overlay error or simply “overlay”, relates to the ability to position successive layers of features in superposition accurately enough to produce working devices with a high yield. Overlay should, in general, be achieved within a few tens of nanometers in today's sub-micron semiconductor devices, down to a few nanometers in the most critical layers. Other performance parameters such as critical dimension (CD or line width) also should be optimized and made uniform across the substrate, to ensure good yield and performance of the manufactured devices. To achieve good performance in such parameters, the substrate should be stable and flat during the patterning step. Typically the substrate is held on a substrate support by a clamping force. Conventionally the clamping is achieved by suction. In the latest lithography tools using extreme ultraviolet (EUV) radiation, the patterning operation is conducted in a vacuum environment. In that case, the clamping force is achieved by electrostatic attraction.